This invention relates to the measurement of strain and more particularly relates to the measurement of strain using optical techniques.
It is known to measure the strain and deformation of objects using extensometers or electrical resistance strain gauges. Unfortunately, dynamic systems such as rotating shafts, extreme environments, such as high temperature or corrosive gas environments, and inaccessible locations, such as large structures or small openings have proven difficult applications for these devices.
Strain measurements may also be made using optical techniques which do not require constant contact with the object to be measured nor exposure of testing apparatus components to a severe environment. The known methods utilize optical phenomena such as photoelasticity, moire interferometry, holography, speckle interferometry, heterodyning and target tracking.
In some methods for optical strain measurement, interference effects are exploited which generally make use of the properties of coherent monochromatic light to produce interference patterns correlated to the strain. An advantage of some interference methods is the ability to measure complete two dimensional strain fields For example, holography can be used for out-of-plane displacements, and moire and speckle methods for in-plane displacements.
A diffraction grating may also be placed on a substrate to be measured and illuminated with coherent, monochromatic radiation. The resulting diffraction pattern is then analyzed to determine strain.
Extensive use of optical techniques for strain measurement has been limited, however, by several disadvantages, including sensitivity to vibration, difficulty in removing the effect of rigid body motions, interference due to the natural irradiance of specimens, and the cost of optics and supporting electronics.